Abstract: This research presents a novel methodology to determine runoffwater retention volumes thatallow the design of storage tanks for storm sewer overflows. It is based on the use of theStormwater Management Model (SWMM) to generate hydrographs and runoffpollutographs ofafictional urban basin. Three pollutants (TS, BOD5and TN) are simulated for a given set of rainsand the values taken by a proposed set of characterization variables for the pollutographsobtained are analysed. Correlation and determination coefficients that exist between thedifferent variables are analysed while also performing a multivariate characterization using PCAand cluster analysis. In the case study presented, using IDF curves of the studied city, aprobability of occurrence (Tr) is assigned to the values taken by the proposed characterizationvariables. To assess the impact and identify the most unfavourable pollutographs within the setof selected rains, impact evaluation variables (IEV?s) are established, based on the proposedcharacterization variables and by simulating the discharge to a receiving water body (river withinitial concentration and constantflow). Finally, a storm sewer overflow is simulated, deriving amaximumflow for purification, and dimensioning retention tanks for different fractions of thetotal volume of runoffto control the maximum values of a specific IEV impact evaluationvariable. Taking a design return period Trssd?10 years, the results obtained in the study casewere 146.50 m3/ha imp for a 100% retention of the total runoffvolume and 117.20 m3/ha impfor an 80% retention

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